1. Field
One or more embodiments described herein relate to an image sensor.
2. Description of the Related Art
An image sensor captures an image using the properties of a semiconductor reacting to light. A complementary metal oxide semiconductor (CMOS) image sensor has proven to consume less power than a charge coupled device (CCD) image sensor. Accordingly, CMOS image sensors are used in portable phones, digital cameras, and other battery powered devices.
A CMOS image sensor may include a pixel array and a column parallel analog-to-digital converter (ADC). In devices of this type, an offset or mismatch may occur as a result of differences in physical characteristics between the pixels and the ADC. The offset may increase when the ADC includes a sample-and-hold circuit.
An offset occurring in a pixel may be reduced using correlated double sampling (CDS), which provides an indication of the difference between a pixel signal and a reset signal output from the pixel. However, it is difficult to reduce a column offset occurring in a circuit (e.g., the ADC) outside the pixel array using CDS. The column offset appears as column-fixed pattern noise (CFPN) in an image.
In order to reduce column offset, a fixed voltage may be selectively applied to the ADC. Then, CFPN in an output of the ADC may be measured. At this time, however, additional hardware for applying the fixed voltage is required.